The invention relates to a bone nail for the treatment of upper arm fractures, comprising a hollow shaft adapted to be introduced proximally into the medullary canal and which is slotted at the distal end, a spreading member cooperating with a screw in the interior of the shank and being pulled into the shank by rotation of the screw and spreading the slotted end.
Nails adapted to be introduced into the medullary canal for the treatment of fractures of hollow cylindrical bones are utilized in many different embodiments with almost all hollow cylindrical bones. So-called interlocking nails have proven to be particularly useful which, with the aid of transverse screws, statically fix the segments of the fracture. Shortening of the bone due to loading during the first phase of healing is thereby avoided. In addition, a high degree of torsional stability is obtained with the aid of an interlocking nail. After removal of the bone screws a dynamic load of the fracture is also possible which is known to stimulate the growth process in the environment of the fracture.
In principle, a locking nail also lends itself well to the treatment of upper arm fractures. Owing to the bone screws, however, the danger exists of important nerves being hurt, so that interlocking nails may be used in upper arm fractures only if the site of fracture is situated in a favourable region.
It is therefore the object of the invention to provide a intramedullary bone nail which is especially suited for the treatment of upper arm fractures.
This object is attained in accordance with the invention in that the profile of the shaft in cross section, at least in the slotted region thereof, has alternating elevations and deepenings at least at the inside thereof, an inner thread being formed in the internal elevations outside the slotted region, and the spreading member being formed by the head of an expansion screw, the shank of which cooperates with the inner thread.
The most favourable site for the introduction of an upper arm bone nail is in the proximal region. Anchoring of the bone nail according to the invention takes place in the distal region by means of dowels. To design a bone nail as an expansion dowel at one end thereof is, on principle, known (European patent 0023228). With the known bone nail, the inside diameter at the distal end is steadily reduced in a direction towards proximal through a corresponding thickening of the wall of the hollow shank of the nail. Cooperating with this inner cone is a conical spreading member which is provided with an inner thread having a screw engaging therein. The head of the screw comes to lie against a guide in the interior of the nail shank. The tool for actuating the screw is introduced from the other end of the shank. Thereby, the spreading member may be pulled into the nail shank and, in doing so, spreads the slotted region of the shank radially outwardly, in order to bring it into engagement with the bone wall.
The known nail shows some disadvantages. To arrange a guide in the interior of the nail and form a thickening in the distal region is very expensive from a manufacturing viewpoint. The maximum diameter of the spreading member must not exceed the outer diameter of the nail, because otherwise the introduction of the bone nail will cause problems. Therefore, the maximum available change in diameter is bound to be limited. The path of adjustment of the spreading member to obtain the maximum spreading is relatively short. Therefore, the maximum spreading which may be obtained at all is relatively small. If, with the known nail, the narrow cross sectional area is exceeded by the spreading member, the fastening in the bone canal will be released again. Therefore, an axial limitation must be provided for the spreading member. When loosening the dowel connection, the screw is rotated in the opposite direction. However, a jamming may occur between the nail wall and the spreading member. It may therefore happen that the screw is completely removed by rotation without the spreading member having moved. In such a case, a blow must be exerted therefore on the head of the screw, in order to release the dowel effect. In this connection, care must be taken that the screw remains connected to the spreading member, otherwise the spreading member will remain in the medullary canal when the nail is being removed.